진짜 빼야 되는데. => I really have to reduce (weight).
빼지 마. => Don't do that.
- 빼다: subtract, reduce, withdraw, vacate, etc.
- "2에서 1을 빼다": subtract 1 from 2.
- "살을 빼다": reduce body weight.
- "투자처에서 자금을 빼다": withdraw funds from an investment.
- "방을 빼다": vacate a room.
- 진짜(really) 빼야 되는데(must reduce weight).
- 빼지 마: ~지 마 is a negative imperative form, like "don't (do something)".
The first sentence is missing the object for 빼다 - subtract or reduce what?
It might have been mentioned just prior. My guess is it is reducing weight (살을 빼야 되는데).